This proposal will test several hypotheses relevant to a circuit-level model of Pavlovian fear conditioning that our lab has been developing. As part of this effort, we developed a new procedure for assessing acquired fear in rats based on conditioned enhancement of the RI component of the eyeblink reflex. This general paradigm, which has a half-century tradition at Yale, was originally derived from the learning theory of Clark Hull. Conditioned RI enhancement is especially amenable to detailed analysis because of its neurophysiological and biomechanical simplicity. Importantly, the methods and results translate easily to non-invasive studies of emotion in humans. The proposed experiments test four hypotheses derived from our conceptual model of acquired fear. Over the long term, these kinds of experiments will contribute significantly to a deep understanding, through this simple model system, of the neurobiology of simple associative learning in general and acquired emotions in particular. Such information has obvious neurobiological and health significance. In regard to the latter, mood and emotional disorders (including bipolar disorders, phobias, post-traumatic stress syndrome, depression and suicide, chronic anxiety, possibly borderline personality disorders, attendant drug abuse and eating disorders, and more general consequences of these conditions) are well recognized to rank among the most important health problems facing the nation.